A diagram shows how researchers first severed the nerve connection with the monkeys' arms and then reconnected their wrists via a rerouted connection to a single neuron. The monkeys were able to then move their wrists and play the game shown, which earned them treats. (Source: Chet Moritz et al Nature)

Research are finding that rerouting nerve signals in primates may be surprisingly easy

Eberhard Fetz, a professor of physiology and biophysics at the University of Washington, led the research. The researchers began by paralyzing the nerves leading to the monkeys' arms. They then placed a single wire on a neuron in the monkeys’ neural cortexes. From there they routed the signal to a single neuron implanted in the monkeys' arm muscles. The computer detected a specific firing pattern in the brain neuron and would then signal the neuron in the arm.

The electric "re-routing" working surprisingly well and the monkeys regained control of their wrists. Their new capability was assessed by a simple video game. The game was controlled by the monkeys' wrist motions. By moving their wrists, they could move a cursor onscreen and by moving it to a box on the side, they could earn a reward. With the incentive of the reward the monkeys soon learned to move their wrists, even though the motor cortex neuron was selected at random.

Chet Moritz, a senior research fellow at the University of Washington and coauthor of the researchers' paper states, "We found, remarkably, that nearly every neuron that we tested in the brain could be used to control this type of stimulation. Even neurons which were unrelated to the movement of the wrist before the nerve block could be brought under control and co-opted."

The research is published in the latest online version of the journal Nature.

Most previous research had focused on complex firing patterns. This is because typically even moving one arm muscle results from the firing of multiple neurons in a coordinated pattern. The success of the single neuron approach raises new questions about how exactly the primate nervous system processes signals.

Regardless of the mechanics, the approach works, and Moritz says that it will be very useful as it requires less computing power. In order to apply the new research to paralyzed patients, more work remains to be done. Most importantly, the researchers will have to learn to make multiple rerouted muscles fire coordinately as they would in the body in a complex motion such as walking, or picking up an object.

For this reason, Andrew Schwartz, a professor of neurobiology at the University of Pittsburgh, remains a skeptic of the new efforts. He states, "If your intention is to generate a movement, you have to somehow calculate the effect of all these forces across the arm. It's not just, 'Activate a muscle and the arm goes where you want.' There's a lot of math involved."

Still, the University of Washington Researchers have moved forward to where one neuron controls two different wrists motions with different firing patterns mapped to each motion and another scenario in which two rerouted neurons each controlled a single muscle (direction of motion) and worked together. Also they say one spinal cord cell, rerouted, can activate multiple arm muscles. Moritz states, "Stimulating a single location in the spinal cord will often activate 10 to 15 different muscles in a precise balance."

The risks are also significant. The electrodes wear down over time. Also if they protrude out of the skin, there's major risk of infection and disruption in a normal daily environment. The ultimate goal, the University of Washington researchers say, is miniaturization. Says Moritz, "We think we may be one step closer to low-power, fully implantable systems."

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